Method for compensating the dark current of an electronic sensor having several pixels

ABSTRACT

A method is proposed for compensating for the dark current of an electronic sensor having a plurality of pixels with an individual dark-current response, radiation for producing an image signal (BS) being directed from a beam source ( 3 ) onto a detector arrangement ( 4 ) containing the sensor, and a recording being taken with different clock-out rate (v) (integration time) by reading the detector signal (S) present in the pixels of the sensor, in which, before the beginning or after the end of the recording with radiation, the sensor is read out without radiation with at least two different clock-out rates (v 1 , v 2 ) and at least two dark-current signals (DC 1 , DC 2 ) are thereby picked up for each pixel, in which the dark-current signals (DC 1 , DC 2 ) that have been read out are then used to calculate a dark-current value (DV) of individual pixels of the sensor as a function of the clock-out rate (v), and in which a correction is then made, using the dark-current value associated with each pixel, to the detector signal (S) with the dark-current signal superimposed on it in order to calculate the picture value (PV).

The invention relates to a method of compensating for the dark currentof an electronic sensor having a plurality of pixels. In this case,radiation for producing a picture signal is directed from a radiationsource onto a detector arrangement containing the sensor, and aplurality of recordings are taken with different recording time(integration time) through reading out the detector signal present inthe pixels of the sensor. A method of this type can, in particular, beused to make dental panorama or cephalometric tomographs with an X-raydiagnosis instrument that contains a rotation unit with a beam sourceand, arranged diametrically thereto, a detector arrangement having anelectronic sensor, preferably a CCD sensor.

EP-A-0 279 294 discloses the use, in dental X-ray diagnosis technology,of a beam-sensitive (CCD) sensor in panorama or cephalometric X-raytomographs instead of a film with an amplifier sheet, and the electronic“reconstruction” of the function of the moved film by a special type ofoperation of the sensor (TDI: time delay and integration), bycorrespondingly clocking forward the sensor's charge packets produced bythe illumination while new charges are being continuously added.

DE-A-19 525 678 describes a method and a device for adjustingpicture-generating values in an X-ray that produces a panorama X-ray. Adisadvantage in this case is that the spatial dependency of the darkcurrent in relation to the individual pixels, which is due to themanufacturing process, is not taken into account. Although a timedependency is implicitly provided by adjusting the signal-to-noiseratio, the described arrangement nevertheless assumes an ideal sensorwhich works without defects in the pixels.

In design terms, X-ray sensors generally consist of a plurality of (CCD)elements arranged with minimal separation. The picture signal producedby X-radiation has a plurality of noise signals superimposed on it. Thedominant impairment to the signal is made by the time- andtemperature-dependent generation of charge carriers in the sensor, whichis referred to in the specialist literature “Solid State Imaging withCharge-Coupled Devices”, Albert J. P. Theuwissen, Kluwer AcademicPublishers Dordrecht, Boston, London 1995, pages 92-95 (and pages274/275 as regards TDI operation) as a dark current. The dark-currentsignal which is generated depends both on the sensor used and on theindividual pixels of the sensor; the dark-current signal is thereforespatially dependent [y]. Furthermore, this signal is dependent on theintegration time and therefore also on the variable turning speed of therotation system, and thereby time-dependent [x]. In addition to this,the dark-current signal is also dependent ion temperature and thelifetime-cumulative X-ray dose.

Hitherto, corrections for the dark current have been made by picking updark-current information at the start and end of a recording from thecovered, that is to say unexposed, edge regions of the sensor. Acorrection value is calculated for each row from the data obtained inthis way. The associated correction value, constant over the entire row,is then subtracted from each pixel of a row.

A disadvantage with this type of dark-current correction is thatalthough fluctuations in the integration time, and therefore thetime-dependent variation in the dark current are taken into account, nofluctuations in the individual pixels on a row of the sensor are takeninto account. Significant picture artefacts can therefore occurdepending on the CCD sensor used.

Another known method of correcting for the dark current is to subtract afull dark-current recording. To do this, in addition to the recording ofthe picture, a dark-current recording is made and is subtracted from theradiation recording. Although such a method gives good results, itcannot be used for the panorama and cephalometric recordings mentionedat the start since costs would be disproportionately increased throughthe dark-current picture and the concomitant storage costs additionallyrequired. Furthermore, the recording procedure would be slowed by therequired delay made necessary by the generation of position pulses.

WO-A-8 910 037 discloses a method and a device for compensating for thedark current and a base-value shift in the voltage of a CCD unit, inwhich a control process is used whose manipulated variable is derivedfrom CCD cells arranged covered. This procedure is repeated continuouslyfor each row of the linear CCD and the dark-current information of thecovered CCD cells is used for correction of CCD cells arrangedphysically at other positions. In this case, the integration unit isoperated with two time constants. To that end, the signals from CCDcells arranged covered are amplified and converted into digital signals.In a comparator unit, the digital values are compared with apredetermined value. If the value coming from the covered cells is toohigh, then the time constant of the integration unit is reduced. If thedigitized value approaches the optimum, then the integration unit isoperated with a large time constant, which results in faster read-out.It is furthermore proposed to assign the integration units a timeconstant which is so high that the amplified output signal remainsapproximately constant during the read-out of a row from the CCD unit.It is nevertheless possible to switch over the time constant of theintegration unit from a first to a second position with smaller timeconstant, in order to implement the control loop, when the situation isfar from the optimum dark-current compensation, as is the case forexample when warming up the instrument or in the event of a significantvariation in ambient temperatures.

A disadvantage here is also that only the CCD cells arranged covered areused for information about the dark-current profile and for the requireddark-current compensation, rather than the CCD cells themselves actuallyused for recording the picture. Furthermore, the time taken to comparethe digital values with a predetermined value at the start of eachpicture row leads to a delay in which no picture data are picked up.

The object of the present invention is to obtain an improved way ofcompensating for the dark current.

According to the invention, before the beginning or after the end of arecording with radiation, the sensor is read out without radiation usingat least two different clock-out rates (integration times), and at leasttwo dark-current signals are thereby picked up for each pixel. Thedark-current signals that have been read out are then used to calculatea dark-current value of the individual pixels as a function of theclock-out rate and the calculated dark-current value available for eachpixel is thereby calculated. This dark-current value is lastly used forpicture correction by subsequently using the dark-current valueassociated with each pixel to make a correction to the detector signalwith the dark-current signal superimposed on it in order to calculatethe picture value.

Advantageously, a CCD sensor is driven using this method, in particularthe operation of a two-dimensional pixel matrix in TDI mode beingespecially advantageous.

The sensor may in this case have a plurality of regions spatiallyseparated from one another, between which no signal need necessarily bepicked up. This non-radiation-sensitive region is advantageouslyminimal, approximately of the order of a pixel. It is thereby possibleto produce a sensor composed of a plurality of regions. The response,deviating in particular in the edge region of each region in comparisonwith the central regions, can be corrected using the method according tothe invention.

For calculating the dependency of the dark-current value on theclock-out rate, a computed relationship is produced with the aid of theat least two dark-current signals that are read out, and thedark-current value assigned to a pixel is calculated by means of acomputed relationship corresponding to an inter- or extrapolation as afunction of the actual recording time. The number of double-currentsignals read out for each pixel can be regarded in mathematical terms asa number of support points for ascertaining a computed relationship. Thedark-current response of a pixel is in this case defined by anapproximation, and the dark-current value of a pixel is expressed as afunction of the integration time as a first-order equation, which isdefined by two support points, or as a higher-order fitting function, ifmore than two support points are picked up.

In order to ascertain the integration times of the individual picturecolumns, the times of the clock-out pulses and therefore the clock-outfrequency are picked up and kept. The integration time of a picturecolumn in TDI operation is the sum of the respective last (n)ascertained clock-out times, (n) in this case defining the number ofpixels in the TDI direction. The number of clock-out times depends onthe width of the sensor and therefore on the number of pixels in acolumn. In a fictitious sensor with for example a 66 pixel width (in theTDI direction), 66 integration pulses are obtained. The integrationtimes are calculated for each picture row from the preceding 66integration rows and are stored.

The method is suitable, in particular, for taking a dental X-rayrecording, since in this case a non-constant speed of motion of thesensor leads to different clock-out rates (integration times) of thesensor.

Advantageously, the method according to the invention is used to takedental panorama or cephalometric tomographs with an X-ray diagnosisinstrument that contains a rotation unit with a beam source and,arranged diametrically with respect to it, a detector arrangement havingat least one electronic sensor. During these recordings, the rotationunit is turned at a non-constant speed of motion around the patient'shead.

The detector signals obtained when the sensor is being read out areadvantageously stored in a memory unit. In a subsequent step, correctionto the detector signal with the dark-current signal superimposed on itis made in order to calculate the picture signal in a computer unitconnected to the memory unit. By means of this, the compilation of thecorrected recording after the generation of the uncorrected recording isdecoupled from the actual recording process, so that the recordingitself is not affected by the correction.

In principle, however, it is also possible to pick up the dark-currentsignals before the start of the recording and carry out real-timecorrection, so that picture values can be displayed actually during therecording.

The procedure is explained in more detail with the aid of FIGS. 1 to 5,in which:

FIG. 1 shows an outline presentation of a dental X-ray diagnosisinstrument for picking up panorama tomographs,

FIG. 2 shows a detector arrangement with a sensor having a plurality ofregions,

FIG. 3 shows a dark-current profile made up of the dark-current valuesof a row of pixels of the detector arrangement in FIG. 2,

FIG. 4 shows a first flow chart with the basic principle of theinvention, and

FIG. 5 shows a flow chart involving the use of a memory.

FIG. 1 shows an outline representation of a dental X-ray diagnosisinstrument for picking up panorama tomographs, abbreviated to PANrecordings below. The instrument contains a height-adjustable supportcolumn 1 holding a rotation unit 2 that supports, on one side, an X-raysource 3 with a primary diaphragm 6 and, diametrically with respect toit, on the other side, a detector arrangement 4. 5 denotes a retainingand positioning device for the head, using which the patient's head canbe fixed in a defined position in known fashion. The structure andpossible adjustments of the rotation unit and the retaining andpositioning device for the head are known and are described, forexample, in EP-0 229 308.

The detector arrangement 4 contains three X-ray-sensitive CCD elements 7to 9 (FIG. 2) which have a specific width (B) and length (L) and arearranged at a small distance one above the other. For PAN recordings,the CCD width (x direction) is typically 5 to 10 mm, and the sensorlength (y direction) is in all 150 mm. The TDI direction, in which thecharge packets are transported within the CCD, is indicated by an arrow.A plurality of pixel rows are arranged next to one another in the widthdirection.

FIG. 3 represents the dark-current profile actually existing for a rowof pixels of the CCD elements 7 to 9, with the aid of the dark-currentvalues DC of each individual pixel of the CCD elements 7 to 9. Thedark-current values of the individual pixels can differ considerablyfrom one another, and so the profiles can differ greatly from oneanother. Due to manufacture, high dark-current values exist especiallyin the edge region, since the sawing of the sensor material causeschanges in the material. Discontinuities therefore arise in the regionof the junctions between two neighboring CCD elements 7, 8, 9.

As can be seen from the flow chart according to FIG. 4, the acquiredpicture information, in the form of the detector signal S, consisting ofthe integrated picture signals PS with the dark-current signals DC, isread out and sent to a computer 10. At the same time, the associatedclock-out rates are measured and kept. As mentioned above, before thestart of a recording or after a recording, at least two data records arepicked up from the unexposed sensor with defined clock-out rates v₁, v₂.The information obtained in this way is subjected to a calculation byapproximation, a dark-current value DV calculated for each pixel beingin this case determined. In conjunction with the integration times,time-dependent correction values are ascertained and are made availableto the computer 10 in order to compensate for the dark current. Thecomputer 10 then delivers picture values PV which are improved in termsof dark-current response and, in known fashion, are processed to form apicture that can be reproduced on a monitor in known fashion.

In FIG. 5, the flow chart is refined by providing a memory unit 11 inwhich the picture information, consisting of the picture signal PS andthe dark-current signal DC, are stored as a detector signal S with theclock-out rate v being picked up. The memory 11 also stores thedark-current signal DC1, DC2, belonging to each pixel, for the at leasttwo recordings with different clock-out rates v₁, v₂, which are used tocalculate the time-dependent dark-current value DV in order to correctthe detector signal S. Through the clock-out rate v, a directrelationship with the integration time is arranged, since for a highclock-out rate v the integration time is low, and vice versa. Theclock-out rate itself has a relationship with the rate of motion of thesensor.

In the computer 10, the dark-current value DV is calculated as afunction of the integration time, the dark-current signals DC1, DC2being employed to produce a first-order equation and the dark-currentvalue DV being ascertained by inter- or extrapolation. If the number ofrecordings without radiation with different clock-out rates v₁, . . .v_(n) increases, then the number of support points increases so that ahigher-order mathematical equation can be produced.

In order to correct the detector signal, the dark-current value DVassociated with the clock-out rate v is then calculated using themathematic equation EQ for each pixel, the dark-current value DV beingsubtracted from the detector signal S. The picture value PV calculatedin this way is displayed on a monitor 12 and, if appropriate, stored inthe memory 11. The picture value can, of course, also be printeddirectly or used in a different way.

What is claimed is:
 1. A method for compensating for the dark current ofan electronic sensor having a plurality of pixels with individual darkcurrent responses, comprising: irradiating the electronic sensor;reading detector signals (S) generated by pixels of the sensor at aclock-out rate (v); reading at least two dark current signals (DC1, DC2)for each pixel, the dark current signals corresponding to clock-outrates (v₁, v₂); calculating a dark current value (DV) for each pixel atclock-out rate (v) using the dark current signals (DC1, DC2) read at theclock-out rates (v₁, v₂); and correcting the detector signals (S) usingthe dark current value (DV) associated with each pixel to determinepicture values (PV).
 2. A method according to claim 1, wherein theelectronic sensor from which the measurement signals are read out is aCCD sensor.
 3. A method according to claim 1, wherein the sensor has atwo-dimensional pixel matrix and is driven in TDI mode.
 4. A methodaccording to claim 1, wherein the detector signals (S) are generatedusing a sensor divided into a plurality of regions spatially separatedfrom one another.
 5. A method according to claim 1, wherein forcalculating the dependency of the dark-current value (DV) on theclock-out rate (v) with the aid of the at least two dark-current signals(DC1, DC2) that are read out, a computed relationship is produced, andthe dark-current value (DV) assigned to a pixel is calculated by meansof an inter- or extrapolation corresponding to the computed relationshipas a function of the actual clock-out rate.
 6. A method according toclaim 1, wherein the detector signals (S) represent a dental X-ray.
 7. Amethod according to claim 1, wherein dental panorama or cephalometrictomographs are taken using an X-ray diagnosis instrument which containsa rotation unit having the beam source and, arranged diametrically withrespect to it, the detector arrangement having at least one electronicsensor.
 8. A method according to claim 1, wherein the detector signals(S) obtained when reading out the sensor are stored in a memory unit,and correcting the detector signals (S) with the dark-current signal(DC) superimposed on it to determine the picture values (PV) is carriedout in a subsequent step in a computer unit connected to the memoryunit.
 9. A method according to claim 1, wherein the at least twodark-current signals (DC1, DC2) are read before reading the detectorsignals (S), and a dark-current value (DV) is calculated during therecording with the aid of the clock-out rate (v), and the detectorsignals (S) are corrected by the dark-current value (DV) and the picturevalues (PV) thus obtained are displayed and/or stored.